The simplest solution is to stay away from cars that have networking ability.

Which basically means you are going to stay away from any car manufactured in the past 10 years.

Yep. My daily driver is 16.

Steering and brakes should work even if there's no electricity.

A backdoor on your current non-self-driving car can disable brakes and suddenly pull the steering wheel to the left as soon as you reach 65 mph.

It can pull the steering wheel, but I'm pretty sure the ratios are selected so that I can still overpower it. The steering wheel is still mechanically connected to the front wheels.

And it surely can't disable brakes. It can disable ABS or force it to cycle, turn off power assist, and apply electric parking brake, but brake pedal is still connected to the brakes through some old school hydraulics.

ABS is able to disengage your brakes even while the pedal is fully depressed, the old-school hydraulics are connected to ABS that can release the pressure so that the brakes disengage - that's what it does during normal operation. If it would intentionally release the brakes and keep them open instead of "pumping", there's nothing you can do.

The same applies for power assist - it's quite powerful; steering at high speed is possible but quite hard if it simply turns off; however, if it would be actively working against you (and do it suddenly without any warning), it's likely that you wouldn't be able to turn it back until you'd already smashed into the opposite lane.

put your car in neutral and turn off your engine, see what happens. You can still steer and brake, but for a sharp turn or down an incline you would be surprised how hard it is without servo steering and the vacuum servo.

Probably drives a 1967 Pontiac GTO.